First - if you want to use the GUI, avoid doing operations on the command line unless it's absolutely required. In some cases CLI changes are erased as of the next, on demand, configuration change. Other possible changes can break OMV's DB or the Debian install itself. If it can be done in the GUI, as a general rule, it's best to do it in the GUI.
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Using Remote Mount, to open the network share on the DNS-323, is usable GUI approach.

To set up Remote Mount, you'll need a username and password that can access the music share on the DNS-323. If you can access, and write files to and from the share, from your current PC, use the username and password that you currently use to log onto your PC ,as the username and password in Remote Mount. Otherwise, root or admin and the password you use to configure the DNS-323 may work. (See the DNS-323 Doc's for the appropriate admin username and password, or other user access choices. You will need "write" access to the music share.)

To set up Remote Mount,
Mount Type: use the default SMB/CIF
Name: Something to help you identify it. ( DNS-323-music )
Server: Use the IP address of the DNS-323
Share: Use the name of the share as it appears on the network.
Username and password - see the para above.

If Remote Mount connects successfully, you'd see a new file system under, Storage, File Systems. It will appear to be something similar to this-> /srv/0a420521-b74f-4215-bb39-6f46927722d8 with a Label that's the same as the name you entered in Remote Mount. (The file system shown will be CIF)

At this point, you'll have what appears to be a local file system. For all functional purposes it is local. You can set the path to the media by coping and pasting in the full path into plex. /srv/0a420521-b74f-4215-bb39-6f46927722d8 Substitute your directory.
( You may have to add a slash "/" at the end of the path similar to /srv/0a420521-b74f-4215-bb39-6f46927722d8/)
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It may be necessary to give a system user "plex" access to this folder. If you change the permissions on this folder from, Group - root to Group - users, then add "plex" to the users group that might do it. I'm not sure about this. I don't use plex so I'm fuzzy on the initial set up.

I'll be out of town until next weekend. If you need help, perhaps others will chime in.